Abstract: The aim of this study is to investigate the bearing characteristics of micropile group foundations in plateau mountainous areas under coupled horizontal and vertical loads (combined V-H loads), and to uncover the law of interaction between these loads. On the basis of field tests, FLAC^3D was employed to numerically analyze three single load cases (pure vertical compression, vertical uplift, and horizontal load), and to simulate combined V-H loads on micropile group foundations with various load ratios. Results reveal distinct behaviors: 1) Compression and horizontal loading tests exhibit gradual variations, underscored by significant pile cap effects, whereas pullout tests demonstrate abrupt changes. 2) Under combined V-H loads, horizontal loading reduces vertical bearing capacity. Higher horizontal load proportion yields larger pile resistance moment and ultimate friction resistance along pile side, eliminating abrupt changes in the curve of load versus uplift displacement. A critical ratio (n) of vertical load to horizontal load exists, equaling 3.9 under combined compression and horizontal loads and 0.76 under combined uplift and horizontal loads. Beyond these ratios, horizontal bearing capacity weakens or strengthens accordingly. 3) Under combined V-H loads, horizontal bearing capacity varies as a quartic function of load ratio inverse, while vertical bearing capacity varies quadratically. The yield envelope and the unidirectional ultimate load perpendicular line enclose a space divided into “failure” and “safety” zones, distinct from unidirectional load conditions. Optimal solutions exist under load coupling, maximizing the bearing capacity of foundation in all directions.

Key words: micropile group foundation, joint vertical-horizontal loading, bearing capacity of pile foundation, prototype test, numerical simulation, plateau mountainous area, power transmission project